Since Free-D is a UDP based protocol, it's connectionless. Reality Field doesn't need to know the IP of the sending device, only the port. If you have multiple Free-D trackers, each one should be set with a different ID.
You can input the Free-D port right under where the LONET IP is, in the Input panel in the bottom right.
Be warned--every manufacturer puts their own spin on Free-D. The axis may come in wrong or inverted. You may need to play with the axis mapping to make it correct.